Data (e.g., a “1” or a “0”) is stored in a 1T memory cell as a voltage level. A “1” is stored as a high voltage level which can decrease due to leakage. A “0” is stored as a voltage level of zero volts which can increase due to leakage. The 1T memory cell requires a periodic refresh to maintain the voltage level stored in the cell. In many applications, a memory chip uses a ring oscillator to control when the refreshes occur. The frequency of a signal generated by a typical ring oscillator decreases with increasing temperature because of CMOS device characteristics. However, the memory cell leakage increases with temperature. As the temperature increases, refresh using a conventional oscillator can occur less frequently than necessary to maintain the voltage level stored in the memory cell. Thus, the oscillator needs to be designed to support the high temperature refresh rate at the expense of more current.
Proportional to absolute temperature(PTAT) voltages and currents are used in temperature monitoring circuits. The monitoring circuits either detect a specific temperature or output a voltage and/or current that increases with temperature. The temperature monitoring circuits can use a PTAT and an inverse PTAT, where the crossing point is a desired temperature. A conventional method of generating PTAT voltage is to use a delta Vbe generator circuit.
Referring to FIG. 1, a block diagram of a circuit 10 is shown. The circuit 10 is a delta Vbe generator circuit that can generate a PTAT voltage VREF. The voltage VREF is described by the following equation 1:                     Verf        =                  Vbe1          =                                                    n                ·                k                            q                        ·                          ln              ⁡                              (                                                                            n                      ·                      k                      ·                                              ln                        ⁡                                                  (                          B                          )                                                                    ·                      T                                                              q                      ·                      A                      ·                      Is                      ·                      R                                                        +                  1                                )                                      ·            T                                              Eq        .                                   ⁢        1            where T is the absolute temperature in Kelvin, n is the emission coefficient, k is Boltzmann's constant, q is the charge of an electron, Is is the theoretical reverse saturation current, A is the smaller of the areas of diodes 12 and 14, B is the ratio of the areas of the diodes 12 and 14, and R is the resistance of the resistor 16. The resistance R generally has a positive temperature coefficient. The emission coefficient n is related to the doping profile and affects the exponential behavior of the diodes 12 and 14. The value of n is normally approximated to be 1.
The voltage VREF is proportional to the temperature T, ln(T), and 1/R(T). Also, a current I is generated equal to Vt*ln(B)/R which is proportional to temperature since R has a positive temperature coefficient and Vt=k*T/q. The voltage VREF is generated by using a voltage across a diode with the bandgap current I flowing through the diode. The circuit 10 has the following disadvantages: a complex relationship between temperature and the voltage VREF (i.e., the voltage VREF is a function of T, ln(T), and ln(1/R(T)); the value of the voltage VREF is limited when the bandgap current I is also used to generate a PVT compensated voltage; and a larger value for the voltage VREF requires a higher current I.